Endurance activities, such as mountain biking, long-distance running, ski touring, hiking, and military operations can be strenuous. Therefore, proper and continuous hydration is an important consideration for individuals during such activities. Hydration systems have been developed to address this need. Typical hydration systems include specialized backpacks equipped with a flexible bladder and a drinking tube so that a sufficient amount of water can be carried and consumed efficiently. With typical hydration systems, the precise amount of water remaining in the bladder during an activity is unknown, leading to potentially inconvenient or dangerous situations. Currently, the amount of water remaining is determined by “experience” (e.g., feeling the weight), by inspection (e.g., visually checking the bladder), or by using a commercially available flowmeter. These approaches each have drawbacks. For example, the weight of the hydration pack felt by the user may not necessarily indicate the amount of liquid remaining in the bladder, as hydration packs often are used to carry gear related to the outdoor activity, which may vary in weight from one excursion to the next. Inspecting the bladder may provide the user with a sure indication of the amount of liquid remaining, however, such inspections require stopping the activity and opening the hydration pack, which is often inconvenient. Although flowmeters do provide some indication of the amount of liquid consumed, the current commercial offerings have reliability and accuracy issues due to their designs.